The Federal Communications Commission plans to designate 100 MHz of spectrum for small cell use, which would go a long way in encouraging the deployment of a dense layer of mobile broadband capacity for our smartphones to romp around in. But there’s a catch: carriers don’t just get to buy this spectrum and lock it into their networks — they have to share it with the government agencies already occupying it.

The FCC’s plan is part of a larger, and quite controversial, proposal first put forth by the President’s Council of Advisors on Policy and Technology (PCAST) to clear 1000 MHz of airwaves for mobile broadband use. That’s far more than the Obama administration is aiming for in its broadband plan, but PCAST’s recommendations all come with the sharing caveat: instead of booting the feds out of their airwaves, the public and private sectors must find a way to coexist.

FCC Chairman Julius Genachowski

The FCC is chipping away at the recommendation one spectrum band at a time, and it’s choosing to start with the 3.5 GHz band. In his announcement Wednesday, commission chairman Julius Genachowski said he would take formal steps by the end of the year to open the 3.5 GHz band for small cell use. While Genachowski never mentioned the word “sharing” in his statement, Ars Technica confirmed with FCC officials that the plan was to make 3.5 GHz shared between its current government radar use and wireless carriers.

The 3.5 GHz band would be ideal for small cell deployments. By creating a separate band for the tiny little networks, carriers wouldn’t have to worry about interference with their primary macro networks. Rather than try to wrap big tower-based macro cells around small cells using the same frequencies, a dedicated band means micro- and pico-cells could be placed directly under the macro umbrella. Each of those small cells would have the same capacity as their larger counterparts, so adding dozens of small cells beneath a single macro cell would boost its capacity enormously.

Also, higher frequencies don’t propagate as far as lower ones, which is why carriers have always sought to acquire lower band spectrum whenever possible. Since small cells, by definition, have much tinier coverage areas, they’re ideally suited for such high-frequency spectrum.

Don’t expect carriers to fall all over themselves to get this spectrum, though. The operators would rather own their licenses outright than share them with anyone, and the big carrier trade group CTIA has said that spectrum sharing in many cases simply won’t work. There are two different ways to share the airwaves — divvying them up by time or by geography — and according to CTIA, both approaches present problems.

In the case of small cells, carriers need that capacity most in densely populated areas where numerous customers are competing for bandwidth within a limited space. So if the government’s radar facilities are geographically distance — out in the desert, say — then spectrum sharing could work. If they’re at a busy metropolitan airport, that’s another story altogether.

The same goes for timesharing. If the government only uses those frequencies in the wee hours of the night, then carriers could easily turn their small cells off as traffic on the network is at its lowest point. The problem is if the Feds need their airwaves during the peak hours of the day, which is when carriers need them most as well.

We’ll see more details emerge as the FCC starts its deliberations, but even if a spectrum-sharing compromise can be worked out at 3.5 GHz, don’t expect the carriers to embrace this deal. They don’t want to set a spectrum sharing precedent, so they’re likely to lobby the government to clear out as much of this spectrum as possible for their own exclusive use.